Continuous wave medium frequency signal transmission survey procedure for imaging structure in coal seams

ABSTRACT

Instrumentation and procedures for detecting geological anomalies occurring in layered coal formations. The instrumentation comprises a medium frequency continuous wave narrowband FM transmitter and receiver pair. Two instrument configurations are disclosed with one being a portable instrument for use with an existing mined area and another being a downhole instrument for insertion into boreholes in unmined areas. Survey procedures are provided to detect anomolies through signal attenuation, path attenuation and signal phase shift. Continuity measurements provide data to determine the existance of anomalies. Tomographic techniques are employed to provide a visual image of the anomaly. Computer aided reconstruction techniques provide such visual images from the generated data.

This is a continuation-in-part of co-pending application Ser. No.483,264 filed on Apr. 8, 1983, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to instrumentation and procedures fordetecting geological anomalies in coal seams and more specifically tocontinuous-wave medium frequency radio imaging techniques combined withcomputer aided reconstruction to provide graphic radiogenic images ofseam anomalies.

2. Description of the Prior Art

Coal seams or deposits occurring in layered formations have beendistorted by many different types of geological mechanisms. Differentialcompaction occurring in surrounding layers causes faults, twists androlls to occur in the seam. Ancient streams have washed coal from bedsleaving sand and rock deposits. These deposits, known as fluvial channelsand scours, can cause washouts and weak roof. Such seam distortions androck deposits are physical barriers to mining equipment. Two types ofunderground mining techniques are extensively used in the coal miningindustry. One type, referred to as room and pillar, or continuous miningcan mine around many of these barriers. The continuous mining techniqueis less expensive and requires less manpower. For example, set-upgenerally requires three shifts of eight people. Continuous mining,however, produces only approximately 300 tons per shift. Longwallmining, the other widely used technique, is much more efficient inuniform coal beds. This method yields production rates averaging 1500tons per shift.

In the United States, the Mining Safety and Health Administrationrequires that retreating, rather than advancing longwalls be used. Onthe other hand, in Europe, advancing longwalls are extensively used.Retreating longwalls are set up to mine in the direction of the mainentry, whereas advancing longwalls mine away from the main entry.Continuous mining techniques are employed to set up the retreatinglongwall. From the main entry, two entry ways are mined at right anglesto the main entry and on either side of the longwall panel. These entryways, the head-gate entry and tail-gate entry respectively, extend thelength of the longwall panel. At the end of the panel, a crosscut ismade between the head gate and tail gate entries. The wall of thecrosscut facing the main entry is the longwall face. The longwallmachine is set up along the face with a heading towards the main entry.As the longwall moves forward, the roof caves in over the mined outarea. A barrier block of unmined coal is left at the end of the run tosupport the roof over the main entry.

The high yield of longwall mining makes it economically advantageous touse where a long panel can be mined. A typical longwall panel containsfrom 500,000 to one million tons of coal. The initial investment andset-up cost of longwall mining are high. Equipment cost averages manymillions of dollars. Longwall set-up requires thirty days minimum, atthree shifts per day with twelve to fourteen men per shift. Thus, set-upexpenses are very large as a result and to achieve the low costproduction advantage of the longwall method a uniform coal seam isnecessary to ensure a long production run. Seam anomalies such asfaults, washouts, interbeddings and dikes can cause prematuretermination of the longwall production run. In many instances, longwallsbecome "ironbound" after encountering an anomaly. Removal of such"ironbound" equipment requires blasting which can damage equipment andexposes miners to extreme danger. Accordingly, if seam anomalies couldbe detected and analyzed in advance of mining, the mining techniquescould be planned for minimum production cost. Where the survey disclosesa long continuous coal seam, the low cost longwall technique can beemployed. If barriers to longwall mining are discovered the mineengineering department can use continuous mining to mine around thebarriers, or the anomalies can be removed, for example by fracking orblasting.

Geological surveys for potentially productive coal formations use manywell known procedures. These procedures employ a wide variety oftechnologies. Satellite imaging and photography provide global data foruse by mine geologists. However, because of the broad overview of thedata are of no value in determining the mineability of a coal seam.Macrosurvey (foot prospecting) of surface strata and outcrop featuresenable geologists to forecast formation characteristics based upon priorknowledge. Surface based seismic and electromagnetic wave propagationprocedures are extensively used in geophysical surveys for valuabledeposits including oil and gas. These microsurvey techniques, however,are not reliable in examining the detailed structure of a coal seam.

Various microsurveying in-seam seismic techniques are currently employedto yield useful data concerning seam anomalies. A technique underdevelopment in Europe comprises firing shots from sixteen points into ablock of 120 geophone groups, each consisting of thirty-six geophones.Computerized processing of the seismic data results in the detection offaults. To date, the procedure requires placing charges at five footintervals and requires the installation of extensive cabling. Seismictechniques are primarily intended for advancing, rather than retreatinglongwalls. Further, this method has not proven to have the capability ofresolving channel sand anomalies, especially for partial washouts andsmaller, less significant anomalies, nor can they detect roof/floor rockconditions. The emerging of the surface based spectral magnetotelluricmethod with controlled sources may have the capability of seeing intothe earth's crust. This method appears to be useful in detecting majorfaults in layered formations, but cannot resolve detailed seamstructure.

Downhole drilling has been used to probe longwall blocks. A ten-twelvehole pattern drilled six-hundred feet into the panel provides samples ofthe coal in the seam. This method, however, has the disadvantage ofcovering only a small percentage of the block. Because of this limitedcoverage this technique is not useful to detect and resolve seamanomalies that may exist in the seam between the boreholes. Surface coredrilling and logging remains the most reliable source of seaminformation. Core sampling provides useful data in mapping stratifiedmedium. Logging enables probing of the formation in the vicinity of thedrill hole. None of the currently used logging methods can detect andresolve seam anomalies that may exist in the seam between the bore-holesover distances greater than about fifty feet. In-seam horizontaldrilling can detect seam anomalies, but is subject to the same coveragelimitations of vertical drilling. Horizontal drilling, additionally, isvery expensive, averaging twenty cents per ton of coal produced.

Electromagnetic technologies have been investigated in an attempt toprovide a geophysical method to see within the coal seams. Conventionaland synthetic radar techniques have been reported in the literature.Because of the high frequency of the radar, it is exceedingly useful ininvestigating the geological structure in near proximity to theborehole. Deep seam penetration, however, requires very high transmitpower in order to maintain any sort of useful resolution. This isbecause high frequency signals are attenuated very rapidly with distancein the seam. Accordingly, present radar methods cannot see deep into theseam.

Publications by R. J. Lytle, Cross Borehole Electromagnetic Probing toLocate High-Contrast Anomalies, Geophysics, Vol. 44, No. 10, Oct. 1979;and Computerized Geophysical Tomography, Proceedings of the IEEE, Vol.67, No. 7, July 1979, have described a method of imaging coal seamsusing continuous wave (CW) signals. His method proposed only tomographicimaging between nearby boreholes. The method of Lytle had limited rangeand resolution, because of the limited spatial measurements that couldbe taken using downhole probes. To satisfy the requirements fortomography, Lytle used a higher frequency range, thus achieving lessrange. Further, the conductivity of rock was found to be much greaterthan the conductivity of coal. Where the difference conductivity(contrast) is large, the tomography algorithm will diverge rather thanconverge, resulting in no image.

A study conducted by Arthur D. Little, Inc. for the U.S. Bureau of Minesinvestigated continuous-wave medium-frequency signal propagation incoal. The results, published by Alfred G. Emslie and Robert L. Lagace,Radio Science, Vol. II, No. 4, April 1976, dealt with the use ofelectromagnetic waves for communication purposes only. Additionally,errors may be present in the wave propagation equations employed. UnitedKingdom Pat. No. 1,018,188, issued to Kaiser, discloses a method fortesting various media utilizing high frequency radio waves. A welllogging method and apparatus is disclosed in EPO Patent Application No.0 105801, assigned to Schlumberger Limited. The method is not directedto deep seam penetration and imaging, but is used to obtain conductivityand dielectric measurements proximate to a borehole.

Other electromagnetic techniques suffer similar range and resolutionproblems. None of the prior art recognized the existance of a coal seamtransmission window in the 300-800 kHz range. Accordingly, none of theprior art achieved a long range, high resolution imaging of geologicalanomalies.

BRIEF DESCRIPTION OF THE INVENTION

Accordingly, it is an object of the present invention to provideinstrumentation and procedures for in-seam and surface imaging of coalseam anomalies with a range sufficient to image an entire longwallpanel.

It is a further object of the present invention to provideinstrumentation and procedures to image coal seam anomalies withresolution sufficient to detect faults, full and partial washouts,fluvial channel sand scours, dikes and interbeddings.

It is a further object of the present invention to minimize productioncosts by surface mappng of fault directions, and providing longwallheadings where appropriate.

It is another object of the present invention to minimize productioncosts by in-seam imaging of longwall panels, after the panel headingshave been developed.

An additional object of the present invention is to mitigate oreliminate hazards to miners resulting from unexpected geologicalanomalies.

It is another object of the present invention to measure coal seamheights.

It is a further object of the present invention to provide detailedin-seam imaging of the seam in advance of the longwall.

It is a further object of the present invention to predict roof falls.

It is a further object of the present invention to verify predictedanomalies by signal strength measuring means.

Briefly, a preferred embodiment of the present invention includes acontinuous-wave medium frequency transmitter with FM capabilities andequipped with a directional loop antenna, a continuous wave mediumfrequency receiver equipped with a directional loop antenna and capableof accurately measuring and recording the received signal amplitude andphase shift of the transmitted signal, and data processing means forproducing a pictorial representation of the coal seam from the raw datagenerated. Both the transmitter and the receiver are portable and aredesigned in two configurations: a cylindrical configuration, referred toas a sonde, for insertion down boreholes in a coal seam, and a portable,or entry configuration adaptable for in-seam use.

The invention further includes survey procedures for imaging structuresin coal seams. Two methods of seam imaging are provided with procedureselection dependent on terrain and seam depth. Surface based seamimaging with downhole continuity instruments is expected to be used inmoderately shallow beds with good surface drilling conditions. In-seamimaging with tomographic techniques will be used when a clear picture ofthe seam structure is required. Combination techniques, utilizing bothdownhole and in-seam measurements may be used for improved resolution ascircumstances dictate.

The preferred embodiment utilizes continuous wave medium frequency (MF)signals to achieve high resolution imaging of geophysical anomalies incoal seams with relatively low output power. The medium frequency rangeis generally defined as being between approximately 300 KHz andapproximately 3 MHz. Because the coal seam is bounded above and below byrock with a differing conductivity, at certain signal frequencieselectromagnetic energy becomes trapped and will propagate over greatdistances. This transmission window, or coal seam mode is excited by thetuned loop antennas employed in the preferred embodiment causing the MFsignals to travel several hundred meters in the coal seam. Seamanomalies create regions with different electrical constitutiveparameters relative to the coal. This electrical contrast between thecoal and the anomalous structure gives rise to the imaging method. Thecontrast will change the wave propagation constant in the region wherebythe wave received on the far side of the region can be analyzed todetermine structure between the transmitter and receiver.

The MF in-seam continuity and tomography instruments further employ FMsignals with a narrow occupied spectrum bandwidth. The receivinginstruments detect and measure the signals with phase-locked-loop (PLL)techniques. PLL receivers extend the signal detection threshold wellinto the noise, thus enhancing operating range. The continuity imagingprocedure is used where relatively large electrical contrast between thecoal seam and anomalous structure is present. Tomography is applicablewhen a small electrical contrast exists. Tomography instrumentation canimprove resolution by making more spatial measurements, thus overcomingthe inherent radar range limitations.

The downhole procedure will require a drilling plan that will enable themedium frequency signals to propagate in the seam between boreholes. Inthis procedure, a plurality of holes are drilled on either side of theseam. The transmitter and receiver probes are inserted into theboreholes on opposite sides of the seam and signal attenuation ismeasured across the seam. A series of data points is generated byvarying the location of the transmitter and receiver across the seriesof boreholes. Signal attenuation, path attenuation, and phase shift aremeasured and compared with calculated values to determine if seamanomalies are present. Additionally these data can be reconstructed bycomputer assisted imaging techniques to provide a pictorialrepresentation of the seam. When a fault is detected additionalboreholes are drilled bisecting boreholes in the original drilling planand further readings are taken to localize the fault. The in-seamimaging technique is carried out in a similar manner to the surfacebased imaging technique except the transmitter and receiver instrumentsare located in the head and tail gate entries adjacent to the seam.

Additionally, the instrumentation can be used to improve mining safetyby detecting coal seam fire headings. Fire in the seam affects the coalseam's conductivity and will thus be detected in the same manner asanomalies are detected. Where a fire is known to exist the in-seamdetection methods are used to localize it so it can be controlled. Inanother application, communications with trapped miners can beestablished by equipping the miners with small receivers ortransceivers. By drilling in the suspected area of the trapped miner,the downhole instrument can excite the coal seam mode and be used tocommunicate with the trapped miner.

It is an advantage of the present invention that graphicalrepresentation of coal seam anomalies are developed by the imagingtechnique.

It is another advantage of the present invention that production costscan be minimized by selecting the appropriate mode of mining the coal.

It is a further advantage of the present invention that the imaging canbe carried out using a minimum of equipment and a minimum of boreholes.

It is a further advantage of the present invention that imaging can beaccomplished using a relatively low transmitter power.

It is a further advantage of the present invention that mining safetycan be improved through the detection of geological anomalies in theworking phase.

It is yet another advantage that partial washouts caused by fluvialchannel sand scour can be detected by the present invention.

It is a further advantage that roof/floor rock conditions can bedetermined by the present invention.

It is yet another advantage of the present invention that theinstrumentation can be used to communicate with trapped miners, thusincreasing mining safety.

It is a further advantage that coal seam fire headings can be determinedwith the present invention.

It is a further advantage of the present invention that coal seamheights may be measured.

It is a further advantage of the present invention that detailed seamimages can be obtained in advance of the longwall.

It is yet another advantage of the present invention that roof falls canbe predicted.

It is a further advantage of the present invention that in-seamverification of anomalies can be performed.

These and other objects and advantages of the present invention will nodoubt become obvious to those of ordinary skill in the art after havingread the following detailed description of the preferred embodiments asillustrated in the various drawing figures.

IN THE DRAWINGS

FIG. 1 is a front elevational view of an in-seam receiver for use in thepresent invention;

FIG. 2 is a front elevational view of an in-seam transmitter for use inthe present invention;

FIG. 3 is a cut away view of a downhole probe for use in the presentinvention;

FIG. 3a is a front elevational view of a downhole probe fitted with aborehole probe centralizer;

FIG. 4 is a schematic representation of the modular components of thedownhole receiver and transmitter probes of FIG. 3;

FIG. 5 is a idealized cut away view of a coal seam, showing the locationof transmitter of FIG. 2 and receiver of FIG. 1 for in-seam tomography;

FIG. 6 is an idealized cut away view of a coal seam showing boreholesadjacent to the coal seam with downhole probes and surface equipment inposition;

FIG. 7 is an idealized top plan view of a coal seam showing a roof rockimaging method;

FIG. 8 is an idealized cut-away view of a coal seam showing a multipleinstrument survey method;

FIG. 9 is an idealized cut-away view of a coal seam showing surveyprocedures utilizing combinations of in-seam and downhole equipment withvertical boreholes and horizontal drillholes;

FIG. 10 is an idealized top plan view of instruments and a procedure forverification of an in seam anomaly; and

FIG. 11 is a schematic view of an alternative embodiment of a downholeprobe of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a portable continuous-wave medium-frequency (CW MF)receiver for in-seam tomographic imaging. As used herein, tomography isa generic term to denote an electromagnetic process of imaging structurein a coal seam using medium frequency (MF) signals. The imaging processmay or may not use the tomography algorithm. The receiver is designatedby the general reference character 10 and is of the single conversion,superheterodyne type with a frequency range of 300 to 800 kHz, tunablein five kHz increments. Receiver 10 is designed to automatically measurefield strength of a transmitted signals, and to convert this measuredfield strength into a digital signal for subsequent data processing.Receiver 10 includes a tuned looped antenna 11, a distance loggingkeyboard 12, a magnetic tape recorder 13 and a field strength display14.

FIG. 2 illustrates a continuous-wave medium-frequency transmitterreferred to by the general reference character 20. The transmitter 20 isa class B transmitter with a frequency range of 300 to 800 kHz, tunablein five kHz increments. The output power of transmitter 20 is twentywatts. This is sufficient to provide a range of at least 1550 feet at520 kHz, depending on the medium. Transmitter 20 is equipped with atuned loop antenna 21. The transmitter antenna 21 and the receiverantenna 11 may be constructed, for example, by sandwiching apredetermined number of turns of wire between a pair of flexible elasticstrips of plastic material, and securing the strips in a circle. Theplastic strips can temporarily deform to allow passage through tightquarters, and may be formed of a flexible plastic sold under thetrademark "Lexan". The transmitter 20 and receiver 10 are constructed ina configuration adaptable for in-seam use as later described herein andillustrated in FIG. 4. In addition to the continuous wave signalcapability of receiver 10 and transmitter 20, the transmitter 20 isfurther designed to generate and transmit narrowband frequencymodulation (FM) signals, and the receiver 10 is further designed toreceive narrowband FM signals and to demodulate those signals. Thiscapability allows for phase shift measurements which yields additionaluseful data in detecting and imaging the geological anomalies. For suchmeasurements, a reference signal must be sent from the transmitter 20 tothe receiver 10. This signal is sent through fiber optics cable 22 asshown in FIG. 5. Fiber optics cable 22 is coupled to receiver 10 throughreference cable connector 24, and is coupled to transmitter 20 throughconnector 26.

In-seam imaging requires access immediately adjacent to the coal seam.In situations where the head and tail gate entries have not been cut, asin the case of advancing longwall mining, or where the bed is moderatelyshallow with good surface drilling characteristics, surface basedimaging employing downhole probes can be used. FIG. 3 illustrates thegeneral configuration of a downhole probe, or sonde and referred to bythe general reference character 30. The probe 30 is of a cylindricalconfiguration for insertion down a standard size borehole. Probe 30includes an outer hollow structural cylinder 37 of a diameter of abouttwo and one quarter inches. Cylinder 37 may be constructed of a varietyof materials, and in the preferred embodiment is of a radiolucentmaterial such as fiberglass. Rotatably mounted within cylinder 37 is asupport frame 40. Support frame 40 is a solid cylinder with alongitudinal trough 42 ending at a pair of opposing flat faces 43.Mounted on each of the flat faces 43 is a modular circuit boardincluding antenna heading control 44, modem 45 and either a receiver 46or a transmitter 47 depending on whether receive or transmit capabilityis desired of the probe. Located in longitudinal trough 42 areconductors of a tuned loop antenna 48. Because of the directional natureof the radio transmissions through tuned loop antenna 48, means must beprovided to mechanically orient the respective antennas of the receiverand transmitter probes to a co-planar alignment. This is accomplished byrotating the frame 40 within the outer cylinder 37 by means of a drivemotor 49. Drive motor 49 is fixed to the proximal end of cylinder 37 andmechanically coupled to frame 40 whereby frame 40 can be rotated througha full 360° within cylinder 37. In the preferred embodiment, power todrive motor 49 is supplied by six nickel-cadmium batteries 50 fixed tothe proximal end of support frame 40, just below drive motor 49.Cylinder 37 is sealed at its distal end by an end cap 51 and at itsproximal end by an end cap 52. End cap 52 contains a standard four pinsocket 53 into which is plugged into a cable 54. Cable 54 is the meansby which the probe 30 is connected to surface equipment and throughwhich data is sent for the imaging process. Electronic surface equipmentfor downhole receiver 30 comprises a signal strength display/recorder, amodem, and a radio transmitter/receiver for sending a reference signalbetween probes when the probes are operating in the FM mode.

To ensure that the probe is centrally located within the borehole, aborehole probe centralizer is fitted to the probe as shown in FIG. 3a.This centralizer includes a proximal end cap 55 which is attached to theprobe proximal end cap 52. End cap 55 has a central aperture throughwhich cable 54 may pass. Sleeve 56 fits snugly around the distal end ofouter cylinder 40 of probe 30. Attached to both end cap 55 and sleeve 56are three to four bands 57. These bands 57 are of a strong flexiblematerial such as thin stainless steel. The bands 57 are prestressed tocurve convexly outward to contact the inside walls of the borehole. Thebands 57 are sufficiently flexible to urge the probe 30 into theborehole's center, and to allow for easy passage of the probe down theborehole. Downhole equipment and surface survey procedures can also beused within the mine, for example, as when unmined seams occur at lowerlevels than the seam being mined. In this situation, boreholes aredrilled from the existing seam to the lower seam and downholeinstruments and procedures are employed.

The preferred embodiment contemplates modular design of the componentsof both the downhole probes 30 and the in-seam instruments 10 and 20. Inthis way cost of production is reduced and repair and servicing issimplified. FIG. 4 is a general block diagram of the components, showingthe modular design.

Table 1 is an equipment identification chart identifying the modularcomponents and Table 2 is a configuration matrix showing the compositionof each downhole probe 30 and in-seam instrument 10 and 20.

                  TABLE 1                                                         ______________________________________                                        Module           Identification                                               ______________________________________                                        A1               Receiver                                                     A3               Transmitter                                                  A10              C.sup.3 Modem                                                A13              Display/Recorder                                             B6               NiCad Battery                                                                 Tuned Loop Antenna                                                            Enclosures                                                   C1               Downhole                                                     C2               Portable                                                     ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        CONFIGURATION MATRIX                                                          Instrument  A1     A3     A10  A13  B6   C1   C2                              ______________________________________                                        Downhole 30                                                                   Transmitter        1      1         1    1                                    Receiver    1             1         1    1                                    Surface                   1    1                                              In-seam (10 or 20)                                                            Transmitter 20     1                1         1                               Receiver 10 1                  1    1         1                               ______________________________________                                    

Examples of equipment specifications are shown in the following Tables3-7. Table 3 describes the general system operating parameters. Tables 4and 5 illustrate the transmitter and receiver specifications,respectively. Table 6 describes the transmitter and receiver antennacharacteristics and Table 7 gives the specifications for the modem.

                  TABLE 3                                                         ______________________________________                                        SYSTEM SPECIFICATIONS                                                         ______________________________________                                        Signal Emmissions:                                                            Type              CW and Narrowband FM                                        Frequency Range   300 to 800 kHz                                              Tuning            5 kHz Increments                                            Peak Deviation    100 Hz                                                      Modulation:                                                                   Frequency Range   200 Hz                                                      Environmental:                                                                Operating Temperature                                                                           -40 to 80 Degree C.                                         Probe             500 PSI                                                     ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Type              Complimentary Class B                                       ______________________________________                                        Module Description                                                                              A3                                                          Frequency Range   300 to 800 kHz                                              Tuning            5 kHz Increments                                            Output Power      20 Watts                                                    Flatness          ± 1/2 dB                                                 RF Load Impedance 50 and 200 Ohms                                             VSWR              3:1 Max                                                     Mixer Oscillator  10.7 MHz                                                    Signal Oscillator 100 Hz                                                      Power Requirements                                                            Operating Voltage 9 to 15 Volts DC                                            Demand Current    3.5 Amperes                                                 Connectors                                                                    Type              Molex                                                       Output Power      2                                                           VCC               1                                                           Ground            1                                                           Fiber Optics Cable                                                                              1                                                           (Reference)                                                                   ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                        RECEIVER                                                                      Type           Single Conversion superheterodvne                              ______________________________________                                        Module Description                                                                           A1                                                             Frequency Range                                                                              300 to 800 kHz                                                 Tuning         5 kHz Increments                                               IF Frequency   10.7 MHz                                                       Source Impedence                                                                             50 Ohms                                                        Sensitivity    0.1 Microvolts for 12 dB Sinad                                 IF Frequency   10.7 MHz                                                       Selectivity                                                                   3 dB BW        200 Hz Mm                                                      70 dB BW       1 kHz                                                          Audio                                                                         Squelched      50 dB Mm                                                       Unsquelched    30 dB (100 UV)                                                 Load           8 Ohms                                                         Frequency Response                                                            3 dB           10, 70, 100 Hz                                                 Voltmeter                                                                     Type           Synchronized with transmitter                                                 local oscillator                                               Range          0 to 360 degrees                                               ______________________________________                                    

                  TABLE 6                                                         ______________________________________                                        ANTENNA                                                                       Type               tuned Loop Antenna                                         ______________________________________                                        Frequency Range    300 to 800 kHz                                             Number of Turns    11                                                         Magnetic Moment                                                               Downhole           8 ATM.sup.2 @ 20 W                                         Portable           8 ATM.sup.2 @ 20 W                                         Heading                                                                       Type               Flux Gate Compass                                                             (Downhole)                                                 ______________________________________                                    

                  TABLE 7                                                         ______________________________________                                        C.sup.3 MODEM                                                                                  Manchester Code Format                                       Type             1b Bit Code                                                  ______________________________________                                        Function                                                                      Power            On/Off                                                       Antenna Position ± 30 of Command                                           ______________________________________                                    

Where there is no immediate access to the coal seam, surface imaging isperformed using the downhole probes 30 inserted into a series ofboreholes around the seam's perimeter. This procedure is represented inFIG. 6.

Imaging of the seam is based upon the differential conductivity of coaland the anomalous structure. The conductivity of rock is several ordersof magnitude greater than the conductivity of coal. The verticalorientation of the transmitting tuned loop antenna produces a horizontalmagnetic field H.sub.φ and a vertical electrical field E_(Z). Thesefields are almost constant over the height of the coal seam. At largeradial distances from the antenna, the fields decay exponentially at arate determined by an effective attenuation constant (α) which dependson losses both in the coal and in the rock, and on the dielectricconstant of the coal. The attenuation rate is expected to increase asthe coal seam thickness decreases, coal seam conductivity increases, andthe relative dielectric constant increases. The attenuation ratedecreases as the rock conductivity increases. The ratio also dependsupon frequency of the transmitted signals, and on water content of theseam. Measured results by Enslie and Lagace indicate an attenuation ratein coal of approximately four dB/100 feet. This forms the basis for thecoal seam continuity imaging method.

The tuned loop antennas excite the natural low loss coal seam modesignal in accordance with the following equation (1):

    H.sub.φ =Mf(rahε.sub.c ε.sub.r σ.sub.c σ.sub.r)                                            (1)

where M is the magnetic moment of the transmit antenna, r is the rangefrom the transmitter to the receiver antennas, α is an attenuationconstant, h equals the seam height, ε_(c) and ε_(r) are the seam androck permitivities respectively, and σ_(c) and σ_(r) are the seam androck conductivities.

The magnetic moment M is itself dependant on transmit power P_(o), andbandwidth BW are represented by equation (2): ##EQU1##

Together, these equations indicate that the tuned loop antennas willexcite the natural coal seam mode magnetic moment and thus provide thehighest magnetic moment resulting in the longest range. Narrow systembandwidth also leads to improved receiver sensitivity and operatingrange. The receiver sensitivity .[.S_(dB) ¹⁰ .]. .Iadd.##EQU2##.Iaddend. for a 10 dB IF signal-to-noise ratio is given byequation (3):

    .[.S.sub.dB.sup.10 =-164+10 Log.sub.10.sup.BW IF=10 Log.sub.10 NF (3).]. .Iadd. ##EQU3##.Iaddend.  where BW.sub.IF is the IF bandwidth of the signals and NF is the noise figure at the receiver.

Equation (3) thus shows that sensitivity increases as bandwidthdecreases. The results of equations (1), (2) and (3) indicate thatcontinuous wave MF signals are optimum for the geophysical surveyinstruments. This is because their narrow bandwidth results in signalpropagation in coal with the lowest attenuation rate (Equation 3) andhighest magnetic moment (Equations 1 and 2). The net result is that theoperating range of the instruments is maximized at a specified outputpower.

The imaging method is dependent on a comparison of the calculated signalstrength with the measured signal strength as determined by the CW, MFinstruments. An analysis of the calculated signal strength is helpful toan understanding of the method. To provide a baseline reading for theimaging method, Equation (1), previously referred to, is used tocalculate the expected signal strength in a particular coal seam. Thisis accomplished by measuring the various parameters at the particularseam and applying these results to Equation (1).

The propagated signal is received in the far side receiver. Because ofeither an increase in the "effective" seam conductivity or a decrease inheight of the seam, wave propagation along a faulted seam path willresult in greater attenuation rate. Thus, by comparing the level of anumber of received signals, path anomalies can be detected.Additionally, the refractive index of MF signals varies slowly in purecoal, thus the signals travel on a straight line path. Propagation incoal takes the form of a parallel plane, transverse electromagnetic(TEM) transmission-line type mode, with the electric field vertical andthe magnetic field horizontal within a planar seam bounded above andbelow by more conductive rock. Seam anomalies such as rock have adifferent refractive index than coal, thereby deflecting the signal pathto a greater degree. The narrowband-FM signals allow for measurement ofthis path attenuation thus providing a second means of detectinganomalies. Finally, phase shift of the FM signal along the path alsoindicates the presence of anomalous formations within the seam.

When greater resolution is desired tomographic, rather than continuityimaging techniques are employed. Tomography results in high resolutionby employing the lowest attenuation-rate frequency and making morefrequent spatial measurements. The attenuation rates are measured ateach spatial orientation of transmitter and receiver and the resultingdata points are analyzed using computer aided imaging techniques toyield a pictorial representation of the anomaly.

To obtain good resolution, the distance between the transmitter andreceiver should be greater than .[.λ_(c) /2.]. .Iadd.λ_(c) /2π.Iaddend.where λ_(c) is the signal wavelength in coal. Thus for the 520kHz transmission frequency of the preferred embodiment, the wavelengthin coal is 97.7 meters. This yields a minimum separation distance of15.56 meters.

The in-seam tomographic imaging method as illustrated in FIG. 5,accordingly is carried out with portable entry receiver 10 andtransmitter 20 in a manner determined to maximize resolution. Tuned loopantenna 21 of transmitter 20 is designed to excite the natural coal seammode azimuthal magnetic field components (H.sub.φ), at a preselectedwavelength in the 400-800 kHz range, thus providing the highest fieldstrength at a given power output. Receiver tuned loop antenna 11 isplaced in a co-planar alignment with transmitter antenna 21 to ensuremaximum field strength. The instruments are separated by a distance ofmore than 15.6 meters for maximum resolution. In the preferredembodiment, a coal seam is 600 feet (200 meters) wide, thus ensuringadequate separation. It can be seen that a coal seam 70 contains withinit a rock/sand barrier 74. This rock/sand barrier 74 will act as aradiogenic mass and cause an attenuation in the transmitted signal.Because tomography employs the lowest attenuation-rate signal frequencyallowing for minimization of transmitter output power, it is importantthat the head and tail-gate entries do not include continuous electricalconductors. Such conductors create secondary magnetic fields that wouldinterfere with received signal level measurements. To obtain adequateresolution in the tomographic in-seam survey a number of data must becollected by taking frequency spatial measurements. The procedure isthus carried out by making a series of received signal levelmeasurements at specific locations (designated as X₀, X₁ . . . X_(n) )in the tail gate entry 76 for each transmitter location in the head gateentry 78. Similarly the transmitter 20 is placed at a series of specificlocations (designated as Y₀ Y₁ . . . Y_(n)) within the head-gate entry78 and the receiver 10 is placed at the corresponding locations in thetail-gate entry 76. At each receiver location in the entry way distancelogging keyboard 12 is used to enter coordinates representing thereceiver's position relative to the transmitter. These coordinates areconverted to digital signals and stored, along with the measuredreceived signal values, on the magnetic tape. In reconstructing thedata, the signal strength measurements can be thus corrected forpositional changes of the transmitter and receiver. The longitudinalspacing of transmitter and receiver locations is determined by therequired image resolution. The resultant received signal data isconverted to digital form and stored on the cassette tape 13 of receiver10 and can be analyzed in a computer based tomography algorithm. Suchcomputer aided imaging algorithms will provide a detailed picture of theseam structure. Computer generated data may also include a print-out ofseam parameters (conductivity, ash content, etc.) and a digital tape foruse by the mines' computer graphics terminal.

Surface based seam imaging equipment is deployed as shown in FIG. 6. Thecoal seam to be imaged is represented by general reference character 80.It can be seen that a fault 82 is present within the seam. In employingthe surface base imaging procedure using a perimeter explorationprotocol a plurality of drill holes 84 are drilled along the perimeterof the longwall panels. In the preferred embodiment two adjacentlongwall panels, of a combined width of 1200 feet and a length of 6000feet can be imaged with a ten-hole drilling plan. Such a drilling planwill detect seam anomalies within the panels. If a fault is detected inthe panel a convergent search strategy is required to determine itsheading. Such a search strategy is carried out by drilling bisectingboreholes within the perimeter to localize the fault. Surface imaging ofthe coal seam is accomplished in a manner similar to the in-seam imagingprocedure. After the series of drill holes have been bored around theperimeter of the longwall panel, the surface probes 30 are inserted downthe boreholes 84 to the depth of the coal seam which has been determinedfrom the core samples. FIG. 6 illustrates a vertical displacement faultwhich results in one end of the coal seam being deeper then the other.Because the signals tend to propagate through the coal. The receiverprobe 30 is placed within the coal seam and not necessarily at the samedepth as the transmitter probe 30. Downhole probes 30, comprising areceiver probe and a transmitter probe are inserted in opposite drillholes 84. Receiver and transmitter antenna segments 48 are aligned in aco-planar manner by means of drive motors 49 located in the end ofdownhole probes 30. Antenna headings are determined and controlledthrough the use of surface control units 85 including telemetry andcontrol equipment and a modem. These units, together with the probecontrol equipment and cabling, are mounted in trucks 86. Continuous waveFM medium frequency (CWMF) transmissions are made in a manner similar tothose made in the in-seam imaging procedure and attenuation rate and/orphase shift are measured. The process is repeated with the receiverprobe 30 and transmitter probe 30 in each of the boreholes 84.

This surface procedure may also be carried out from an undergroundsurface. This situation occurs when a lower coal seam is being exploredfrom an existing mined seam. The existing seam floor then becomes thesurface into which boreholes are drilled for exploration of the lowerseam. The downhole probes 30 are inserted into the boreholes andmeasurements taken in the normal surface exploration manner.

The resultant continuity data are analyzed to determine if areas ofincreased attenuation are present. Such areas indicate fault zones orother geological anomalies. If sufficient boreholes are drilled, thedata can be analyzed using computer-aided tomographic imaging techniquesto yield a pictorial result as with the in-seam procedure.

Additionally, floor/roof rock characteristics can be determined usingcontinuous wave medium frequency continuity measurements. This procedureis analogous to the downhole seam continuity measurements except thereceiver and transmitter probes 30 are positioned above the coal seamceiling to measure roof conditions, and just below the floor to measurefloor conditions. The signals are thus propagated through thesurrounding rock, and signal attenuation is measured. In this way adetermination respecting the type of rock and resulting roof/floorconditions can be made.

Roof rock characteristics can also be determined by placing the in-seamreceiver 10 and transmitter 20 within the tail gate and head gateentries 76 and 78 at very close intervals, for example, on the order oftwenty five feet, and analyzing the resulting data using a backplaneand/or reconstruction algorithm to develop an isopach map of constantattenuation rates. FIG. 7 illustrates a method for imaging roof rockconditions in a developing entry 100, which is mined between theexisting entries 76 and 78. The receiver 10 is placed at a series oflocations (X₀, X₁ . . . X_(n)), for example, within the entry 76 and thetransmitter 20 is placed at a series of locations (Y₀, Y₁ . . . Y_(n))for example, within the entry 78, with the developing entry 100 betweenthe entries 76 and 78. Attenuation rates are determined by propagatingthe medium frequency signals from the transmitter 20 to the receiver 10,and a backplane and/or reconstruction algorithm may be used to analyzethe resulting data. For improved imaging, the intervals betweeninstrument locations may be short, for example, twenty-five feet orless. It may be noted that this method can be used to image a singledeveloping entry 100, or multiple entries 100 between existing entries76 and 78.

FIG. 8 illustrates a survey procedure using a single transmitter 20 andmultiple receivers 10 placed within the coal seam 70. This procedurespeeds up acquisition of the necessary data, and works equally well witha single receiver 10 and multiple transmitters 20 or multiple receivers10 and transmitters 20.

Numerous variations on the in-seam and downhole survey techniques can beutilized for specific purposes. As illustrated in FIG. 9, the downholeprobes 30 may be used in conjunction with an in-seam receiver 10 and/ortransmitter 20 in a crosshole survey method to obtain high resolutionimaging of anomalies that may exist in a virgin local seam 102 next tothe developing entry 100. Backplane reconstruction and/or tomographyalgorithms may be used to analyze the resulting data.

FIG. 9 also illustrates a method for measuring seam heights along ahorizontal in-seam drillhole 104. This method makes use of the crossholemethod using an in-seam transmitter 20, and a downhole-type probe 30which is inserted into the horizontal drillhole 104, which is drilledperpendicular to a transmitter survey line A. The receiver 30 islaterally displaced within the drillhole 104 and signal attenuationreadings are taken at various points for each of a plurality oftransmitter locations designated as Y₀, Y₁, . . . Y_(n). A plot ofattenuation rate versus displacement of the instrument in the horizontalborehole 104 can be analyzed to determine seam heights along thedrillhole 104. While this method is intended for use with coal seams 70which have preexisting entries 76 and 78 or developing entries 100 forpositioning an in-seam instrument 10 or 20, the method can also be usedwith a downhole probe 30 inserted into a plurality of boreholes 108which are in a line perpendicular to the drillhole 104. Also illustratedin FIG. 9 is a method of guided parallel in-seam drilling to image theseam 70 between a pair of horizontal longitudinal drillholes 110. Thedownhole-type instruments 30 are inserted into the drillholes 110 andmultiple readings are made at various transmitter and receiver locationsas with all of the survey procedures. A variation on this procedure isto drill both the longitudinal horizontal drillhole 110 and thetransverse drillhole 104, with radio wave transmission occurringdiagonally between the drillholes 104 and 110. As with all of thesemethods, a vertical downhole instrument may be used in conjunction withone or both of the horizontally displaceable instruments. It may benoted that with modern directional drilling technology, horizontalin-seam drillholes can be drilled from surface drill rigs, thus furtherexpanding the scope of the drillhole survey procedure. The drillholes110 and/or 104 may be drilled, for example, at different heights withinthe seam 70. Any of the foregoing in-seam survey procedures may also beused to image cavings in block caving mining.

Anomalies detected by any of the foregoing survey methods can beverified by a comparison of direct and reflected signals. A transmitter,which may be the inseam instrument 20 or the downhole instrument 30, ispositioned in the seam 70, as illustrated in FIG. 10, to propagatecontinuous wave MF signals towards the anomaly 74. A receiver, eitherthe in-seam instrument 10 or the downhole probe 30 is positionedadjacent to the transmitter to receive a reflected signal from theanomaly 74. A signal from a transmitter/receiver loop antenna 111 ispropagated by a first port of a directional coupler 112 and then to theanomaly 74, which reflects part of the signal. The reflected signalenters a second port of the coupler 112 via the antenna 111, and withinthe coupler 112 a comparison of direct and reflected signal power ismade. The presence of the anomaly 74 will cause an increase in reflectedpower over that reflected by the seam 70.

Once an anomaly has been imaged in a coal panel 70, its effect onlongwall mining can be ameliorated by separating the longwall intopieces, for example a first piece which would avoid the anomaly and asecond piece which would run into it. A take-down room and a set-up roomare mined, using continuous mining techniques, on either side of theanomaly and perpendicular to the longwall heading. The entire longwallis operated until the second part enters the take-down room, at whichpoint this part is taken down and reset in the set-up room. The firstpart of the longwall meanwhile continues to operate and is relinked withthe second part on the far side of the anomaly. An alternative to allowuninterrupted longwall mining of the panel is to fracture the anomaly byany means known in the art, for example, by fracking with high pressureinjections of mud, or high expansion rosins, or by conventionalblasting.

Boreholes on the order of 1700 feet or more are costly and timeconsuming to drill and expensive to case. The lack of casing causes thehole to close after the drill stem is removed. For such boreholes, analternative embodiment of the probe 30, illustrated in schematic in FIG.11, may be utilized. These probes, designated by the reference character120 essentially comprise a loop antenna 48', a motor 49' mechanicallycoupled to the antenna 42' for rotating the antenna 42' about a verticalaxis, a voltage level calibrator circuit 121 and a cable 122 forcoupling to a power and control means 123. The surface power and controlunit 123 includes transmitter and/or receiver circuitry for deliveringsignals to or receiving signals from the loop antenna 48'. Bothelectrical power and intelligence signals are carried from the unit 123to the probe 120, and intelligence signals are carried back to the unit123, via the cable 122. The voltage level calibrator circuit 121generates a precision signal level for calibrating the entire signalpath from the antenna to the control means 123. The probe 120 can beleft in the borehole for extended time periods while a drilling protocolis completed, and may be used at any time as desired. The probe 120 mayalso include a security code lock circuit 124 in order to preventunauthorized use of the probe 100. Typically, the probes 120 will beinserted into the borehole through PVC piping, or they may be inserteddirectly through the drill stem. The pipe or stem is subsequentlyremoved, leaving the probe 120 in place in the borehole.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the disclosure. Accordingly, it is intended that theappended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

In the claims:
 1. A method for detecting geological anomalies inunderground coal seam formations comprisingplacing an FM transmitterhaving continuous wave transmit capabilities in a medium frequency rangeof between approximately 300 KHz to approximately 800 KHz about a coalseam to be analyzed, the transmitter including a tuned-loop antenna forpropagation through said seam; placing a medium frequency FM receiverhaving continuous wave receive capability and including a tuned-loopantenna about said seam remote from the position of the transmitter withsaid seam intermediate to the transmitter and receiver, said tuned-loopantennas of the receiver and transmitter being positioned to be verticalto said seam, said receiver further including measuring and recordingmeans for measuring and recording a plurality of characteristics of saidreceived waves propagated through said seam; exposing said seam to aplurality of transmissions of continuous wave medium frequency waveswith an azimuthal magnetic field component propagated horizontallythrough said seam from the transmitter towards the receiver; measuring aplurality of signal transmission characteristics through said seam;generating a number of data points by locating both the transmitter andreceiver at several points about said seam such that for eachtransmitter location the receiver is placed at a plurality ofpreselected points about said seam; calculating a plurality of expectedsignal transmission characteristics through said seam; and comparingsaid calculated signal transmission characteristics with said measuredsignal transmission characteristics and generating a graphicalrepresentation of said formation thereform.
 2. The method of claim 1whereinthe resulting data are analyzed with a computer based tomographyalgorithm wherein a graphical representation of the coal seam anomaly isproduced.
 3. The method of claim 1 further comprisingdrilling aplurality of first set of boreholes adjacent to one side of said coalseam to be imaged and drilling a plurality of a second set of boreholesopposite to the seam, with the seam intermediate to the first and secondsets of boreholes; sequentially inserting into each of the first set ofboreholes said medium frequency transmitting probe and inserting into aplurality of the second set of boreholes said medium frequency receivingprobe, the receiving and transmitting probes being inserted into saidboreholes to the depth of said coal seam and approximately equal indepth to each other; transmitting medium frequency signals from saidtransmitter probe in each of the first set of boreholes to said receiverprobe in said plurality of the second set of boreholes such that foreach transmitter location signals are sent to a receiver probe at eachof a plurality of said second set of boreholes, and measuring aplurality of signal characteristics at each location; calculating aplurality of expected signal transmission characteristics through saidcoal seam; detecting seam anomalies by comparing said measuredcharacteristics along each transmission pathway with said calculatedsignal characteristics determined by the coal seam composition, and bycomparing path attenuation along the different paths; and analyzing thedata with computer aided imaging techniques to provide representationsof the resultant anomalies.
 4. The method of claim 3 whereintheboreholes are drilled approximately 1200 feet apart.
 5. The method ofclaim 3 whereinseam anomalies are localized by drilling additionalbisector drillholes between the original drillholes where high signalattenuation occurs, and by generating additional data points therefrom.6. The method of claim 1 whereinthe underground formation is a coalformation; and the anomaly is a coal seam fire.
 7. The method of claim 1whereinsaid signal transmission characteristics include signal fieldstrength.
 8. The method of claim 1 whereinsaid transmitter furtherincludes an output-connector means for transmitting a reference signaltherethrough; said receiver further includes an input-connector meansfor receiving said reference signal; said reference signal input andoutput-connector means are coupled to a reference signal transmissionmeans; and said measured signal transmission characteristics include aphase shift determined by transmitting said reference signal throughsaid reference signal transmission means and comparing said transmittedreference signal with said signals transmitted through said coal seam.9. The method of claim 1 whereinsaid measured signal transmissioncharacteristics include signal field strength; and said calculatedsignal transmission characteristics include signal field strength. 10.The method of claim 1 wherein said medium frequency range is betweenapproximately 400 KHz and 800 KHz.
 11. The method of claim 1 whereinsaid tuned loop antennas are positioned to be coplanar.
 12. The methodof claim 1 wherein,at least one receiver is positioned in a downholelocation if a transmitter is positioned in an in-seam location, and atleast one transmitter is positioned in a downhole location is a receiveris positioned in an in-seam location.
 13. The method of claim 1wherein,a plurality of transmitters and receivers are placed about theseam, and a plurality of data points may be generated simultaneously.14. The method of claim 1 wherein,a horizontal drillhole is drilledtransversely into the seam, and a downhole instrument is inserted atvarious points therein.
 15. The method of claim 1 wherein,the geologicalanomaly is a roof rock condition; and the transmitter is placed at aplurality of locations about a first lateral edge of the seam, thereceiver is placed at a plurality of locations about a second lateraledge of the seam opposite to said first edge, and the coal seam and adeveloping entry are intermediate to the transmitter and receiverlocations.
 16. The method of claim 1 wherein,at least a first and asecond horizontal drillhole are drilled into the seam, the transmitterbeing inserted into said first drillhole and the receiver being insertedinto said second drillhole, and said continuous wave FM signals beingtransmitted therebetween for various transmitter and receiver locationswithin said drillholes.
 17. The method of claim 16 wherein,said firstand said second drillholes are parallel to each other and longitudinalalong the seam.
 18. The method of claim 16 wherein,said first and seconddrillholes are perpendicular to each other.
 19. The method of claim 16and further including,at least one vertical borehole and downhole probeassociated therewith, with transmission of continuous wave FM signalsoccurring between the vertical borehole and at least one of thehorizontal drillholes.
 20. The method of claim 1 wherein,the seamanomalies are verified by, (a) placing the transmitter with the seamproximate to the anomaly and placing the receiver adjacent to thetransmitter; (b) coupling the transmitter to a directional coupler andsending a direct signal therethrough towards the anomaly within theseam, and receiving in the directional coupler a reflected signal; and(c) comparing a relative signal strength of reflected and direct signalsin the directional coupler whereby a low ratio of reflected to directsignal strength indicates the absence of an anomaly and a high ratioindicates the presence of the anomaly.